The technical field of this invention is solid state integrated circuit fabrication and, more particularly, methods for programming voltage programmable link structures sometimes called antifuses.
Programmable conductive paths, particularly "links" between two or more distinct layers of conductive materials such as metals, polysilicon or doped silicon are increasingly employed in solid-state integrated circuit fabrication to produce a variety of programmable circuits including, for example, field programmable gate arrays ("FPGAs"), programmable read only memories ("PROMs"), and other programmable electronic devices.
Most typically, such devices are "programmed" by the application of an electrical voltage to breakdown a dielectric disposed between two conductive layers and thereby establish an electrical connection across a region of the device which had previously been an insulator.
To be useful, link structures must remain insulating at the normal operating voltage for solid state devices (e.g., nominally five volts). On the other hand, they must reliably "break down" in response to a programming voltage which is higher than the normal operating voltage, but not so high as to damage other structures on the circuit (e.g., not more than about fifteen volts).
It is believed that there is a three stage process of breaking down the insulating layer and rendering it conductive. During the first high resistance stage of this process, the programming voltage, which may typically be in the range of between 20 and 10 volts, produces a low current with a current density of less than 10 mA/cm.sup.2 for periods of 0.01 to 10 milliseconds. At the end of this time, the conductance increases sharply and in a period measured in nanoseconds a typical link decreases in resistance by some ten orders of magnitude to a value of tens of ohms. This in turn causes a sudden increase in the current. In the final stage, which may last several microseconds, chemical reactions continue until a low resistance link is obtained. The final value of resistance depends on the amount of current passed through the link during the second and third stages. Typically, the current density that yields a ten ohm resistance value is 10.sup.4 A/cm.sup.2 or larger.
In a practical application such as in a field programmable gate array, transistors are deployed to provide the switching current necessary to leave the programmed links in a low resistance state. In one such commercial embodiment, it is necessary for these transistors to provide as much as five milliamps of current which in turn requires that the transistors be very large. Further, a number of these transistors may be in series, requiring a still further increase in transistor width to provide the necessary programming current. As a result of these requirements, there is a considerable waste in silicon area devoted to these programming transistors that could be better used to provide more logic modules to improve the overall functional capability of these arrays.